1. Field of the Invention
This invention generally relates to the use of a pilot signal modulated on a carrier to indicate that the carrier also includes a particular type of data or information, such as stereo or multiphonic related modulation components. In particular, the invention relates to AM stereo systems wherein a carrier wave having stereo related modulation components is provided with a low frequency pilot signal modulation component to indicate the presence of stereo information.
2. Description of the Prior Art
Compatible stereophonic AM radio systems and single side band systems are well known, as illustrated by prior U.S. Pats. Nos. 3,218,393 and 2,020,327. Furthermore, the use of a pilot signal in AM stereo systems to indicate the presence of stereo information in the transmitted signal is also well known, as illustrated by U.S. Pat. No. 3,944,749.
The proposed AM stereo systems of the prior art disclose the incorporation of a single low-frequency pilot tone (such as 15 Hz) which is suitably modulated on the radio frequency (RF) carrier at the transmitter. At the receiver, the pilot tone is detected and used to enable the stereo signal decoder and a stereo indicator lamp. However, such prior art systems are adversely affected by low frequency noise and interference which may cause false stereo channel enabling or stereo indication. For example, co-channel interference can create relatively strong low frequency components which may falsely trigger a 15 Hz stereo pilot signal detector, thereby causing the receiver to produce a false indication of stereo reception.